In recent years, a development of a wavelength-division multiplexing system has been advancing in an optical communication, and recently, an optical communication system has been drawing attention, which has an add/drop (Add/Drop Multiplexer: ADM) function to sort out and utilize only a signal light of a specific wavelength from among plural signal lights at a repeater station being provided in a middle of an optical transmission line, or to add and transmit other signals from this repeater station into the optical transmission line, in addition to perform a transmission/reception between two stations. A device disclosed in Japanese Patent Laid-open No. 2000-183816 is known as an optical add/drop multiplexer utilized in such an optical communication system.
The optical add/drop multiplexer includes an optical demultiplexer which receives a multiple wavelength light from the optical transmission line and demultiplexes it into signal lights of respective wavelengths, and an optical multiplexer which remultiplexes and transmits the signal lights demultiplexed into the respective wavelengths to the optical transmission line, and has two by two channel optical switches for switching optical paths of the signal lights to either (a) or (b) described below corresponding to the respective signal lights demultiplexed by the optical demultiplexer, between the optical demultiplexer and the optical multiplexer.
(a) transmits the signal lights straight to the optical transmission line without dropping them to the repeater station; and
(b) drops the signal lights to the repeater station and simultaneously adds the signal lights transmitted from the repeater station to the optical, transmission line.
The multiple wavelength light inputted to such an optical add/drop multiplexer is firstly demultiplexed into the signal lights of the respective wavelengths by the optical demultiplexer. Further, the optical paths of the respective signal lights are selected to be either the aforementioned (a) or (b), by the optical switches provided in accordance with the signal lights of the respective wavelengths, and only the signal lights of the wavelengths used at the repeater station are dropped at the repeater station. The dropped signal lights are utilized at the repeater station, and the signal lights transmitted from the repeater station to be added to the optical transmission line are sent to the optical multiplexer via the optical switches. The signal lights of the respective wavelengths are remultiplexed at the optical multiplexer and sent to the optical transmission line as the multiple wavelength light.
As the two by two channel optical switch used for the optical add/drop multiplexer, switches using prisms are conventionally known as disclosed in Japanese Patent Laid-open No. 2000-2843 and U.S. Pat. No. 5,436,986.
FIG. 12 shows a conventional optical switch 100 of the two by two channel using prisms. In the optical switch 100, an inputting optical fiber collimator 10 which inputs signals demultiplexed by the optical demultiplexer, and an outputting optical fiber collimator 11 which sends the signal lights to the optical multiplexer for multiplexing are provided to face each other at a predetermined spaced interval. On the other hand, a dropping optical fiber collimator 12 which drops the signal lights to a repeater station is disposed adjacent to the inputting optical fiber collimator 10, and an adding optical fiber collimator 13 which adds the signal lights from the repeater station is disposed adjacent to the outputting optical fiber collimator 11, and the dropping optical fiber collimator 12 and the adding optical fiber collimator 13 are also disposed to face each other at a predetermined spaced interval.
Furthermore, two right-angle prisms 14 and 15 are movably disposed in a predetermined spaced intervals formed between the facing two pairs of collimators. A switching of the above-described optical paths (a) or (b) is conducted by a movement of the prisms. In other words, when the prisms are placed at the positions shown by solid lines in the drawing, a light outputted from the inputting optical fiber collimator 10 is reflected by these prisms and inputted to the dropping optical fiber collimator 12, and a light added from the adding optical fiber collimator 13 is similarly reflected by these prisms and inputted to the outputting optical fiber collimator 11, and thereby, the optical path (b) is formed.
On the other hand, when the prisms are placed at retreat positions shown by dotted lines in the drawing, the signal light outputted from the inputting optical fiber collimator 10 is inputted to the outputting optical fiber collimator 11, and thereby the optical path (a) is formed.
Besides, as a similar two by two channel optical switch, a switch using a reflector instead of the prisms is generally known (for example, refer to Japanese Patent Laid-open No. 2001-133705). It is a switch which reflects an input light with the reflector by disposing the reflector movably between the outputting optical fiber collimator and the inputting optical fiber collimator so as to switch the optical path.
However, the aforementioned optical switches using the prisms or the reflector had problems described below. Conventional optical switches use a catoptric system, and a loss of lights at the time of reflection is inevitable when drop of the signal lights to the repeater station and addition of the signal light from the repeater station are conducted.
Besides, at the time of reflection, a light is reflected in an orthogonal direction by the prisms or the reflector, and a phenomenon (PDL: Polarization Dependence Lass) in which a loss is changed depending on a polarized wave of the light is inevitable when the light is thus reflected in an oblique direction. When such a PDL occurs, a bit error rate in the optical communication becomes high, and as a result, there is a problem of deterioration in the quality of the optical communication.
Furthermore, in the conventional optical switches, it is required to adjust the prisms or the reflector to the optical fiber collimators with high precision in addition to position adjustments between the optical fiber collimators. Especially, when the catoptric system such as the prisms, the reflector, or the like is used, axis deviation of the reflected light becomes large due to an angular misalignment of the prisms or a mirror, and therefore, there is a problem that the alignment of optical parts is difficult.
The present invention is made in order to solve the aforementioned problems, and it is an object of the present invention to provide an optical switch which does not use the catoptric system and easy for alignment. Furthermore, it is an object of the present invention to provide an optical add/drop multiplexer using this optical switch.